The present invention relates generally to Sagnac interferometers or more particularly to gyros, which employ optical waveguides, formed on or in the surface of a substrate such crystaline Lithium Niobate or silica glass.
Optical rotation sensors, such as RLGS (ring laser gyroscopes) FOGS (fiber optic gyroscopes), IFOGS (interferometric fiber optic gyroscopes), RFOGS (resonant fiber optic gyroscopes) and integrated optic gyros are based on the well-known non-reciprocal optical effect known as the Sagnac effect.
A FOG has a fiber optic coil formed on a coil form. Light from a common source is launched into each end of the coil to form CW (clockwise waves) and CCW (counter-clockwise waves). When the FOG is at rest in inertial space, the CW and CCW waves have the same transit time through the coil. When the two waves are coupled out of the coil and superimposed on a detector, they exhibit a near zero phase difference. When the FOG is rotated around its axis of symmetry, however, the two waves no longer have identical transit times, and will exhibit a phase difference that appears as an interference pattern on the detector, that increases with the rate input to the gyro.
The phase difference, and hence, the output intensity that results, is proportional to the rotation rate, as well as to the area enclosed by the fiber optic coil. Analysis of the output intensity generated by the combined light waves at the photodetector, typically by means input signal modulation and output signal demodulation, provides a precise indication of rate and direction of rotation.
Winding lengths of low-loss optical fiber into a relatively small coil creates a large effective area, making it possible for a compact sensor to resolve very small rotation rates. However, the manufacturing cost of the fiber optic coil component for a FOG gyro is considerable and can the cost can exclude their use from some applications. In addition, output bias stability problems due to varying thermal gradients throughout a coil wound on a bobbin or coil form can limit the FOG""s performance. This invention provides an IOG, (integrated optic gyro) and reduces the cost of the waveguide coil for an IOG, further adapting the FOG to mass-production, and provides better thermal control of the coil for enhanced performance. The invention increases the scale factor of an IOG by using at least two substrates, each containing a spiral shaped waveguide.
The area enclosed by the turns within the coil limits the scale factor of an IOG. Integrated optic gyros that use waveguides that are formed on, or in, the surface of a substrate have a sensitivity that is limited by the total area enclosed by the turns of the spiraling waveguide. The area is doubled by the use of a second substrate, and, in addition, a novel method is taught for forming the turns on the substrate.
The spiral coils, formed on the top and bottom substrates, are positioned to be co-axially aligned. The coils are coupled together using one or more optical fiber pigtail connections. The structure of dual substrate coil mounted on opposing sides of a thermally conductive mounting plate is believed to improve the thermal control of the coil over the prior art.